You should give a little more information, specify which operating system you are using, what program you use to listen to the Mod, what kind of music modules are,if they are compacted with some specific compressor etc..

Thank you very much for the suggestion! I think the culprit is the commodity WindowToFront, I remember it crashing WinUAE some time ago, but forgot that. Many commodities are patches, and I read one report, in which WindowToFront was crashing real Amiga computers.